The present invention relates to a method and apparatus for processing of automatic images and, more particularly, but not exclusively to a method and apparatus that allows the automatic detection of changes from images taken of the same place at different times.
In the early days of aerial photography, images had to be pored over painstakingly to find out required information, especially when what was needed was to spot small changes. Today the task has been made easier by the use of automatic image analysis techniques to find candidate differences which are then presented to a human user to make a final identification. Nevertheless the sheer volume of photographic images that can be available in a high precision survey means that even such methods leave a huge amount of work for a human analyst to have to carry out. A high precision survey of a region may in fact photograph the region from a fairly low altitude with an ability to resolve objects of the ten-centimeter order of magnitude, and if a human analyst has to consider every suspect change that the computer locates then the effectiveness of the process is highly problematic.
A further issue is that the aerial photograph is taken from a certain position and a certain angular inclination. An aerial photograph needs to be converted into what is known as an orthophoto before it can be treated as a map. An aerial photograph and an orthophoto or orthoimage may look alike but there are several important differences that allow an orthophoto to be used like a map. A conventional perspective aerial photograph contains image distortions caused by the tilting of the camera and terrain relief (topography). It does not have a uniform scale. You cannot measure distances on an aerial photograph like you can on a map. That is to say, an aerial photograph is not a map. The effects of tilt and relief are conventionally removed from the aerial photograph by a mathematical process called rectification. The resulting orthophoto is a uniform-scale image. Since an orthophoto has a uniform scale, it is possible to measure directly on it, just as with a map. An orthophoto may serve as a base map onto which other map information may be overlaid. Until the issue of scale is dealt with, it is difficult to begin the process of looking for changes since two images taken from even slightly differing positions simply do not show the same thing.
However, even after dealing with the issue of scale, current automatic image processing systems have difficulty in ruling out irrelevant differences between photographs such as those due to naturally moving objects, those due to lighting changes between the times the photographs were taken, those due to failure to carry out accurate matching between the successively taken photographs and the like.
One method of matching images comprises finding the same point on the two images and using that as a reference location. However this does not always work since similar points which are not actually the same can erroneously be chosen. Thus any kind of terrain in which repeating features appear is liable to erroneous matching of similar neighbors.
There is thus a widely recognized need for, and it would be highly advantageous to have, an automatic feature matching system devoid of the above limitations.